The last several years have witnessed an explosion in the understanding of vertebrate development, largely based on work from a few model genetic organisms. The embryonic zebrafish is one of these new models, and many research grants are funded by National Institutes of Health and NIEHS to utilize this model. Studies of the embryology and genetics of zebrafish and the recent identification of thousands of genetic mutations have lead to this dramatic increase in the number of laboratories (now ca 250) using zebrafish to study the basic mechanisms of vertebrate developmental biology. Although zebrafish have become an extremely important research model, relatively little is known about the diseases that affect them when held in captivity. Indeed, severe health problems in zebrafish research colonies could seriously jeopardize many millions of dollars in research funding, and some research facilities have experienced devastating, acute mortalities in their zebrafish colonies. Moreover, persistent, but less severe, infections have plagued several facilities. As with other laboratory animals used in research, it is imperative to conduct studies with disease-free, healthy zebrafish. Unlike rodent models, there are no certified SPF stocks of zebrafish, and the investigators are just now developing sensitive diagnostic tests for the most serious pathogens. The major aim of the present study is to improve the health of zebrafish used in research facilities by elucidation of the pathogenesis and modes of transmission of the most important pathogens in this fish e.g., Pseudoloma (P.) neurophilia, a microsporidium of the central nervous system that is very prevalent in zebrafish, and Mycobacterium spp, the cause of fish tuberculosis. As P. neurophilia is extremely prevalent in zebrafish facilities, the investigators will also investigate its impact on fecundity. The investigators will improve polymerase chain reaction (PCR)-based diagnostic tests for these pathogens. With these tests in hand the applicants will elucidate epidemiological factors of these infections in zebrafish colonies. Results will be quickly distributed to the zebrafish community through Zebrafish International Network, and through peer-reviewed publications. In addition, the investigators have developed a Web-based fish health manual, and new information generated in the present study will be incorporated into this manual.